Fabrication method for device structure having transparent dielectric substrate

ABSTRACT

A semiconductor device has a transparent dielectric substrate such as a sapphire substrate. To enable fabrication equipment to detect the presence of the substrate optically, the back surface of the substrate is coated with a triple-layer light-reflecting film, preferably a film in which a silicon oxide or silicon nitride layer is sandwiched between polycrystalline silicon layers. This structure provides high reflectance with a combined film thickness of less than half a micrometer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of application Ser. No.11/252,632 filed on Oct. 19, 2006, which is hereby incorporated byreference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for forming a semiconductordevice on a transparent dielectric substrate such as a sapphiresubstrate, more particularly to the formation of a reflective film onthe transparent dielectric substrate to enable the substrate to berecognized optically.

2. Description of the Related Art

Semiconductor integrated circuits formed in silicon films grown onsapphire substrates are advantageous for applications in environments inwhich radiation poses a hazard. Such silicon-on-sapphire (SOS)integrated circuits are generally formed by use of conventionalfabrication equipment of the type that creates semiconductor integratedcircuits in semiconductor substrates. In conventional fabricationprocesses, the fabrication equipment often uses optical sensors todetect the position of the semiconductor substrate. The position of asapphire substrate cannot be detected in this way because sapphire istransparent light passes straight through the substrate instead of beingreflected back to the sensor. One known solution to this problem is tocoat the sapphire substrate with a light-reflecting film.

Japanese Patent Application Publication No. 7-283383 and the parent U.S.Pat. No. 5,877,094, for example, describe a sapphire substrate coated onits backside with a layer of polycrystalline silicon (polysilicon) atleast about two micrometers (2 μm) thick, which reflects light and canbe detected optically. Phosphorous ions are also implanted into selectedregions of the polysilicon film to form conductive doped regions thatcan be detected electrically.

One problem with this substrate is that forming a polysilicon layer atleast about 2 μm thick is a time-consuming and therefore expensiveprocess. Moreover, in reflow and other subsequent heating steps, thelarge difference in thermal expansion coefficients between sapphire andpolysilicon may cause the sapphire substrate to warp. Such warpinginterferes with the fabrication process and may lead to the formation ofcracks in the sapphire substrate, particularly if the sapphire substrateis thin, which is the current trend.

Japanese Patent Application Publication No. 11-220114 describes an SOSsubstrate having an optically reflecting polysilicon coating 0.5 μm to3.0 μm thick on its backside. A pattern of cuts is formed in thereflective coating so that the difference in thermal expansioncoefficients does not cause the substrate to warp or crack. Thethickness of the polysilicon coating must be at least 0.5 μm because athinner film would lack the necessary reflectivity, as pointed out inparagraph 0009 of the above disclosure.

Due to the trend toward thinner sapphire substrates, there is acontinuing need for still thinner reflective films.

SUMMARY OF THE INVENTION

An object of the present invention is accordingly to provide asemiconductor device having a transparent dielectric substrate with areflective coating film that can be thinner than 0.5 μm and stillprovide adequate reflectivity for optical detection.

The term ‘semiconductor device’ as used herein refers to an electronicdevice such as a semiconductor integrated circuit chip or to a waferfrom which such electronic devices may be manufactured.

The semiconductor device has a dielectric substrate transparent tolight, a first film disposed on the back surface of the dielectricsubstrate, a second film disposed on the first film, and a third filmdisposed on the second film. The first film and the second film havedifferent reflective characteristics, enabling one film to reflect lightnot reflected by the other film.

The first, second, and third films combine to form a triple-layerlight-reflecting film that has a higher reflectance than theconventional single-layer light-reflecting film and can be made thinnerthan the conventional single-layer light-reflecting film.

One method of fabricating the semiconductor device includes: preparing adielectric substrate that is transparent to light and has a frontsurface and a back surface; forming a first film on the back surface ofthe dielectric substrate; forming a second film on the first film;heating the second film; and forming a third film on the heated secondfilm. Another method of fabricating the invented semiconductor deviceincludes: preparing a dielectric substrate that is transparent to lightand has a front surface and a back surface; forming a first film on theback surface of the dielectric substrate; forming a second film on thefirst film by heating the first film; and forming a third film on thesecond film.

In one aspect of both methods, the second film has a lower refractiveindex than the first and third films.

In another aspect of both methods, the first, second, and third filmshave an aggregate thickness less than 0.5 μm.

In another aspect of both methods, the first film includes polysilicon,the second film includes silicon oxide, and the third film includespolysilicon.

In a further aspect of the preceding aspect, the first film is 42nanometers thick, the second film is 110 nanometers thick, and the thirdfilm is 42 nanometers thick.

In another aspect of both methods, the dielectric substrate includessapphire.

Another aspect of both methods also includes exposing the front surfaceof the dielectric substrate.

Another aspect of both methods also includes forming a fourth film onthe front surface of the dielectric substrate.

The fourth film may include polysilicon.

Another aspect of both methods also includes forming a fifth film on thefourth film.

The fifth film may include silicon oxide.

In another aspect of both methods the dielectric substrate has sidesurfaces; this aspect also includes forming a sixth film covering theside surfaces of the dielectric substrate.

The sixth film may include polysilicon and silicon oxide.

When the second film includes silicon oxide (SiO₂), the abovefabrication processes improve the crystalline structure of the siliconoxide. An attendant advantage is that in further fabrication stepsinvolving etching by hydrofluoric acid, there are fewer crystal latticedefects through which the hydrofluoric acid can invade the silicon oxidefilm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a semiconductor device according to thepresent invention;

FIG. 2 is a graph indicating the reflectance of a wafer with atriple-layer reflective coating as a function of wavelength;

FIG. 3 is a graph indicating the reflectance of a wafer with anothertriple-layer reflective coating as a function of wavelength;

FIG. 4 is a more detailed sectional view showing an example of thestructure of the substrate in FIG. 1;

FIG. 5 is a more detailed sectional view showing another example of thestructure of the substrate in FIG. 1;

FIG. 6 is a more detailed sectional view showing yet another example ofthe structure of the substrate in FIG. 1;

FIGS. 7 to 11 illustrate steps in a fabrication process for thesemiconductor device in FIGS. 1 and 6; and

FIGS. 12 to 16 illustrate steps in another fabrication process for thesemiconductor device in FIGS. 1 and 6.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will now be described with reference to theattached drawings, in which like elements are indicated by likereference characters.

First Embodiment

Referring to FIG. 1, the first embodiment is a semiconductor device orwafer having a silicon-on-sapphire (SOS) substrate 101, the majorsurfaces of which are a front surface and a back surface 101 a. The backsurface 101 a and side surfaces 101 b of the SOS substrate 101 arecovered with reflective films 102, 103, 104 to permit optical sensing bywafer sensing light. The second reflective film 103 has a lowerrefractive index than the first and third reflective films 102, 104. Thefirst reflective film 102 is, for example, a polysilicon film formed onthe back and side surfaces of the SOS substrate 101. The secondreflective film 103 is, for example, a silicon oxide film formed on thefirst reflective film 102. The third reflective film 104 is, forexample, a polysilicon film formed on the second reflective film 103.

Next the determination of the film thicknesses of the reflective filmswill be described.

Let the refractive index of the space through which the wafer sensinglight travels before entering the SOS substrate 101 be n₀, therefractive index of the sensed material be n_(x), and the refractiveindex of the space on the far side of the sensed material, through whichthe light travels if it passes through the sensed material, be n_(s). Inorder for the sensed material to have high reflectance, its refractiveindex n_(x) must be the higher than the space indices n₀ and n_(s).

If the refractive index of the sensed material is higher than the spaceindices and the sensed material comprises a triple-layer film consistingof a first reflective film 102, second reflective film 103, and thirdreflective film 104, a reflectance close to unity can be achieved forthe triple-layer film as a whole if the refractive index of the secondreflective film 103 is lower than the refractive index of the firstreflective film 102 and third reflective film 104.

Let the wavelength of the wafer sensing light be λ and the refractiveindices of the first reflective film 102, second reflective film 103,and third reflective film 104 be n₁, n₂ and n₃, respectively. If thewafer sensing light impinges normal (at a 90° angle) to the frontsurface of the SOS substrate 101, then to achieve still higherreflectance, the thickness d of each film, the wavelength λ of the wafersensing light, and the refractive index n of the film must satisfy thefollowing equation (1) for some integer N:

$\begin{matrix}{{\left( {{2N} + 1} \right)\frac{\pi}{2}} = {2\pi\; n\frac{d}{\lambda}}} & (1)\end{matrix}$

A reflectance as close as possible to unity is achieved when thethicknesses and refractive indices n₁, n₂ and n₃ of all three filmssatisfy this equation (1) and the refractive indices also satisfy therelationship mentioned above (n₂<n₁ and n₂<n₃).

If the wavelength of the wafer sensing light is six hundred fortynanometers (λ=640 nm) and the first and third reflective films 102, 104are polysilicon films, their refractive indices are both 3.80(n₁=n₃=3.80). If the second reflective film 103 is a silicon oxide film,its refractive index at this wavelength is 1.45 (n₂=1.45). In order toachieve the minimum film thickness, N should be equal to zero (N=0). Thethickness of the first and third reflective films 102, 104 can then becalculated from the above equation (1) as d=42.1 nm, while the filmthickness of the second reflective film 103 can be calculated as d=109.8nm. If the first and third reflective films 102, 104 are polysiliconfilms and the second reflective film 103 is a silicon nitride (SiN)film, then its refractive index is 2.02 (n₂=2.02), the film thickness ofthe first and third reflective films 102, 104 is still d=42.1, and thethickness of the second reflective film 103 is d=79.2 nm from equation(1) with N=0.

The reflectance of the wafer as a whole to the wafer sensing light canbe calculated from the following equation (2), where as above, n₀ is therefractive index of the space through which the wafer sensing lighttravels before entering the SOS substrate 101, n₁, n₂, and n₃, are therefractive indices of the first, second, and third reflective films 102,103, 104, and n_(s) is the refractive index of the space behind thethird reflective film 104, assuming that the wafer sensing lightimpinges onto the front surface of the semiconductor substrate at anormal (90°) angle.

$\begin{matrix}{R = \left( \frac{{n_{0}n_{s}n_{2}^{2}} - {n_{1}^{2}n_{3}^{2}}}{{n_{0}n_{s}n_{2}^{2}} + {n_{1}^{2}n_{3}^{2}}} \right)^{2}} & (2)\end{matrix}$

In the above example, as the dielectric substrate is transparent, itsrefractive index may be set equal to the refractive index (n₀) of thespace through which the wafer sensing light travels before entering theSOS substrate 101. Equation (2) indicates that in order to achievehigher reflectance than that can be achieved by a single-layerlight-reflecting film made from a high-index material, the first andthird reflective films 102, 104 should be made from a material with acomparatively high refractive index, while the second reflective film103 should be made from a material with a relatively low refractiveindex.

The refractive index n of each material varies according to thewavelength λ of the wafer sensing light, so from equation (2), thereflectance R of the wafer as a whole also varies according to thewavelength. A plot of the reflectance R of the wafer as a whole versusthe wavelength λ of the wafer sensing light is shown in FIG. 2 for thecase in which the first and third reflective films 102, 104 arepolysilicon films, and the second reflective film 103 is a silicon oxidefilm.

If the wafer must have a reflectance R not less than 0.8 in order to berecognized by the wafer sensing light, the wavelength λ of the sensinglight should be about 640 nm±100 nm. The corresponding thickness of thefirst and third reflective films 102, 104 can be calculated fromequation (1) as d=42.1±6.6 nm (hereinafter referred to as about 42 nm).This can be taken as the allowable thickness range of the first andthird reflective films 102, 104. The thickness of the second reflectivefilm 103 can be calculated from equation (1) as d=109.8±17.2 nm(hereinafter referred to as about 110 nm). This can be taken as theallowable thickness range of the second reflective film 103.

A plot of the reflectance R of the wafer as a whole versus thewavelength λ of the wafer sensing light is shown in FIG. 3 for the casein which the first and third reflective films 102, 104 are polysiliconfilms, and the second reflective film 103 is a silicon nitride film. Ifthe requirement for recognition of the wafer is relaxed to a reflectanceR not less than 0.7, the wavelength λ of the sensing light should againbe about 640 nm±100 nm. The thickness of the first and third reflectivefilms 102, 104 can again be calculated as d=42.1±6.6 nm (about 42 nm)from equation (1), the thickness of the second reflective film 103 canbe calculated as d=79.2±12.4 nm (hereinafter referred to as about 80nm), and these can the taken as the allowable thickness ranges of therespective films.

Next, the structure of the SOS substrate 101 will be described. The SOSsubstrate 101 is fabricated by depositing various films on a sapphiresubstrate. In this embodiment, the SOS substrate 101 may be of any oneof the following three types.

The first type of SOS substrate 101, shown in FIG. 4, comprises asapphire substrate 105 (dielectric substrate) and a device formationfilm 106 (a fourth film) formed on the sapphire substrate 105. Thesapphire substrate 105 in FIG. 4 is six hundred micrometers (600 μm)thick; the device formation film 106 formed on the sapphire substrate105 is 100 nm thick. The device formation film 106 can be made fromsilicon, which is the material from which transistors are typicallymade.

The second type of SOS substrate 101, shown in FIG. 5, comprises asapphire substrate 105, a device formation film 106 formed on thesapphire substrate 105, and a silicon oxide film 107 (a fifth film)formed on the device formation film 106. The sapphire substrate 105 inFIG. 5 is 600 μm thick, the device formation film 106 formed on thesapphire substrate 105 is 100 nm thick, and the silicon oxide film 107formed on the device formation film 106 is 10 nm thick. The deviceformation film 106 may again be made of silicon. The SOS substrate 101shown in FIG. 5 has the advantage that the silicon oxide film 107protects the device formation film 106 during wafer processing stepsperformed prior to the formation of circuit elements, resulting in lessvariation in the electrical characteristics of the circuit elements.

The third type of SOS substrate 101, shown in FIG. 6 comprises thesapphire substrate 105, device formation film 106, and silicon oxidefilm 107 described above, and a protective film 108 (a sixth film)covering the side surfaces of the device formation film 106 and siliconoxide film 107 and the back surface of the sapphire substrate 105. Thesapphire substrate 105 in FIG. 6 is 600 μm thick, the device formationfilm 106 is 100 nm thick, the silicon oxide film 107 is 10 nm thick, andthe protective film 108 is 700 nm thick. The device formation film 106may again be made of silicon. The protective film 108 may be made from acombination of a silicon nitride film and polysilicon. The SOS substrate101 in FIG. 6 has the same advantages as the SOS substrate 101 in FIG.5, and the additional advantage that the sides of the device formationfilm 106 can be protected from invasion by hydrofluoric acid, thuspreventing flaking of the device formation film 106 and silicon oxidefilm 107. Furthermore, this structure can prevent unwanted diffusionduring doping steps in the formation of circuit elements.

Any one of the three types of SOS substrate 101 described in FIGS. 4 to6 can be used, according to the needs of the particular application.

Because of its triple-layer structure, the light-reflecting film of asemiconductor device according to the first embodiment of the inventioncan be thinner than a conventional single-layer light-reflecting film.Semiconductor chips can be fabricated by coating part or all of a waferwith a light-reflecting film according to the invention, forming circuitelements on the semiconductor substrate and interconnecting them byusing conventional semiconductor fabrication equipment, and then dicingthe wafer into individual chips. If the wafer sensors in the fabricationequipment illuminate only selected parts of the wafer, the triple-layerlight-reflecting film only has to cover the selected parts. For example,the triple-layer light-reflecting film may cover only the peripheralparts of the wafer. Then after the wafer is divided into chips, none ofthe chips includes any portion of the light-reflecting film, so thethickness of the semiconductor chips can be further reduced.

Next, a process for fabricating a semiconductor device of the above typewill be described with reference to FIGS. 7 to 11.

Among the SOS substrates shown in FIGS. 4 to 6, a fabrication processusing the SOS substrate shown in the FIG. 6 will be described. Fornumerological consistency, the component parts are numbered as shown inFIG. 7. The SOS substrate 201 in FIG. 7 comprises a transparentdielectric sapphire substrate 205, a device formation film 206 formed onthe sapphire substrate 205 as a silicon film, a silicon oxide film 207formed on the device formation film 206, and a protective film 208formed on side surfaces of the sapphire substrate 205, the deviceformation film 206 and the silicon oxide film 207, and the back surfaceof the sapphire substrate 205.

Next, the fabrication process of the SOS substrate 201 will besummarized. First, a sapphire substrate 205 is obtained and a siliconfilm is formed thereon by chemical vapor deposition (CVD). Next, thepart of the silicon film near the interface with the sapphire substrate205 is transformed into amorphous silicon by an implantation process.Then the silicon close to the interface is crystallized by heating in anoxygen atmosphere to form the device formation film 206, and the siliconoxide film 207 is formed by oxidizing the remaining silicon filmsimultaneously. Next, the circumference is coated with a polysilicon CVDfilm; then the circumference is coated with a silicon nitride film.Next, the silicon oxide film 207 is exposed and the protective film 208is formed to complete an SOS substrate 201 of the same type as shown inFIG. 6.

The SOS substrate 201 can have various structures other than thestructure described above. For example, a substrate comprising thesapphire substrate 205 and the device formation film 206, or a substratecomprising the sapphire substrate 205, the device formation film 206 andthe silicon oxide film 207 can be used. A silicon-on-insulator substratecomprising fused silica instead of sapphire is also usable instead of anSOS substrate, but the following description will continue to assume anSOS substrate.

As shown in the FIG. 8, a first reflective film 202 is formed to coverall sides and surfaces of the SOS substrate 201. The first reflectivefilm 202 is a film comprising polysilicon formed by CVD, and has a filmthickness adjusted to 42 nm.

Referring to the FIG. 9, a second reflective film 203 is formed to coverthe first reflective film 202. The second reflective film 203 is asilicon oxide film formed by CVD, and has a film thickness adjusted to110 nm. Next, the second reflective film 203 is heated in a nitrogen(N₂) atmosphere at 950° Celsius for 20 minutes. The CVD process used toform the second reflective film 203 forms a silicon oxide film with poorcrystallization, containing much vapor, which could be easily invaded byhydrofluoric acid during wet etching steps. The subsequent heating step,however, readily eliminates the vapor from the silicon oxide film,giving the silicon oxide film an improved crystalline structure thatprevents invasion by hydrofluoric acid.

Referring to the FIG. 10, a third reflective film 204 is formed,covering the second reflective film 203. The third reflective film 204is a polysilicon film formed by CVD, and having a film thicknessadjustable to 42 nm by the time the light-reflecting film is needed forwafer detection. That is, if the thickness of the third reflective film204 will be reduced by fabrication steps carried out after the formationof the three films 202, 203, 204, the third reflective film 204 mayoriginally be made thicker than 42 nm in order to obtain the desiredfilm thickness of 42 nm at the time of wafer detection.

Referring to the FIG. 11, the silicon oxide film 207 of the SOSsubstrate 201 is exposed by removing the first, second, and thirdlight-reflecting films 202, 203, 204 from the front surface of thesubstrate. The first, second, and third light-reflecting films may beremoved by dry etching.

The above process fabricates a semiconductor wafer device according tothe second embodiment of the invention. After the triple-layerlight-reflecting film has been formed, semiconductor integrated circuitdevices can be fabricated by using conventional semiconductor ICfabrication equipment with optical wafer sensors to form any desiredcircuitry in and on the device formation film 206, and then dicing thewafer into chips.

In a variation of the second embodiment, the triple-layerlight-reflecting film does not cover the entire back surface of thewafer. In particular, if the optical wafer sensors illuminate onlyselected parts of the wafer, the triple-layer film can be removed fromthe other parts of the wafer to reduce the thickness of the chips.

In the fabrication process of the second embodiment, the heating stepimproves the crystalline structure of the second light-reflecting film.Furthermore, the problem of unintended detachment of the thirdlight-reflecting film can be avoided. This problem occurs when atriple-layer light-reflecting film is formed by sequentially depositinga first light-reflecting film, a second light-reflecting film, and athird light-reflecting film made from polysilicon, silicon oxide andpolysilicon, respectively, on the back surface of a dielectric substrateby CVD. In this method, in subsequent steps using hydrofluoric acid, theacid reacts with the silicon oxide film material of the secondlight-reflecting film, thereby invading the silicon oxide film. If theinvasion proceeds far enough, eventually the third light-reflecting filmbecomes detached. By avoiding this problem, the second embodimentmaintains the desired optical properties of the light-reflecting filmand prevents detached fragments of film from contaminating thefabrication equipment.

Next, a semiconductor device fabrication process according to a thirdembodiment of the invention will be described with reference to FIGS. 12to 16. Steps similar to steps in the second embodiment will not bedescribed in detail.

Referring to FIG. 12, an SOS substrate 301 comprising a sapphiresubstrate 305, a device formation film 306, a silicon oxide film 307,and a protective film 308 is obtained. A detailed description of thisstep will be omitted, as the SOS substrate 301 is similar in structureand fabrication to the SOS substrate described in the second embodiment,or any of the SOS substrates described in the first embodiment.

Referring to FIG. 13, a first reflective film 302 is formed to cover allsides and surfaces of the SOS substrate 301. The first reflective film302 is a polysilicon film formed by CVD. Part of the first reflectivefilm 302 will become a silicon oxide film as described below. To allowfor a doubling of the thickness of this part when the polysilicon isoxidized, the thickness of the first reflective film 302 is reduced to100 nm.

Referring to FIG. 14, a second reflective film 303 is formed coveringthe first reflective film 302. The second reflective film 303 is formedby heating the first reflective film 302 at 950° Celsius in an oxygenatmosphere for an appropriate length of time to oxidize substantiallythe outer 58 nm of the first reflective film 302. The oxidizationprocess approximately doubles the thickness of the oxidized material,creating a second reflective film 303 substantially 110 nm thick. Asecond reflective film 303 formed in this way has a better crystallattice structure than a silicon oxide film formed by CVD, and canbetter prevent invasion of hydrofluoric acid in subsequent wet etchingsteps. The remaining part of the first reflective film 302 issubstantially 42 nm thick.

Referring to FIG. 15, a third reflective film 304 is formed covering thesecond reflective film 303. This step will not be described in detailbecause it is similar to the corresponding step described in the secondembodiment.

Referring to FIG. 16, the silicon oxide film 307 of the SOS substrate301 is exposed. This step is also similar to the corresponding step inthe second embodiment, and will not be described in detail.

This completes the fabrication of a semiconductor wafer device accordingto the third embodiment of the invention. As in the second embodiment,semiconductor integrated circuit devices can be fabricated by formingdesired circuitry in and on the device formation film of the SOSsubstrate, using conventional semiconductor fabrication equipment withoptical wafer sensors, and then dicing the wafer into individual chips.As noted in the second embodiment, before the circuitry is formed, thetriple-layer light-reflecting film can be removed from parts of thewafer not illuminated by light from the optical wafer sensors, to reducethe thickness of the chips.

The third embodiment has effects similar to those of the secondembodiment, and the additional advantage of reduced cost, compared tothe second embodiment, because the second light-reflecting film isformed by heating in an oxygen atmosphere, so one CVD step can beomitted from the process described in the second embodiment.

The invention is not limited to a silicon-on-sapphire substrate. It isapplicable to a semiconductor device with any type of transparentdielectric substrate, and may include any type of semiconductormaterial.

Those skilled in the art will recognize that further variations arepossible within the scope of the invention, which is defined in theappended claims.

1. A method of fabricating a semiconductor integrated circuit chipcomprising: preparing a wafer including a dielectric substrate that istransparent to light and having a front surface and a back surface, afirst film formed on the back surface of the dielectric substrate, asecond film formed on the first film, a third film formed on the secondfilm, and a fourth film formed on the front surface, the second filmhaving a lower refractive index than the first and third films; formingcircuitry in and on the fourth film, and dicing the wafer into chips;and illuminating the third film by sensing light for detection of thewafer, during said forming, wherein the first, second and third filmsare formed to cover only selected parts of the dielectric substrate, sothat the chips produced by the dicing do not include the first, secondand third films.
 2. The method of claim 1, wherein the selected partsare peripheral parts of the wafer.
 3. The method of claim 1, whereinsaid preparing the wafer comprises: preparing the dielectric substrate;forming the first film on the back surface of the dielectric substrate;forming the second film on the first film; heating the second film; andforming the third film on the heated second film.
 4. The method of claim1, wherein said preparing the wafer comprises: preparing the dielectricsubstrate; forming the first film on the back surface of the dielectricsubstrate; forming the second film on the first film by heating thefirst film; and forming the third film on the second film.
 5. The methodof claim 1, wherein the first, second, and third films have a combinedthickness less than 0.5 μm.
 6. The method of claim 1, wherein the firstfilm comprises polysilicon, the second film comprises silicon oxide, andthe third film includes polysilicon.
 7. The method of claim 1, whereinthe first film comprises polysilicon, the second film comprises siliconnitride, and the third film comprises polysilicon.
 8. The method ofclaim 1, wherein the dielectric substrate comprises a sapphiresubstrate.
 9. The method of claim 1, wherein the fourth film comprisessilicon.
 10. The method of claim 9, further comprising forming a fifthfilm on the fourth film.
 11. The method of claim 10, wherein the fourthfilm comprises silicon and the fifth film comprises silicon oxide. 12.The method of claim 1, wherein the dielectric substrate has sidesurfaces, the method further comprising forming a sixth film coveringthe side surfaces.
 13. The method of claim 12, wherein the sixth filmcomprises polysilicon and silicon nitride.
 14. A method of fabricating asemiconductor integrated circuit chip comprising: preparing a waferincluding a dielectric substrate that is transparent to light and havinga front surface and a back surface, a first film formed on the backsurface of the dielectric substrate, a second film formed on the firstfilm, a third film formed on the second film, and a fourth film formedon the front surface, the second film having a lower refractive indexthan the first and third films; forming circuitry in and on the fourthfilm, and dicing the wafer into chips; and illuminating the third filmby sensing light for detection of the wafer, during said forming,wherein the first film comprises polysilicon, the second film comprisessilicon oxide, and the third film includes polysilicon, and wherein thefirst film has a thickness of substantially 42 nm, the second film has athickness of substantially 110 nm, and the third film has a thickness ofsubstantially 42 nm.
 15. The method of claim 14, wherein the dielectricsubstrate includes sapphire and the fourth film includes silicon, themethod further comprising: forming a fifth film on the fourth film, thefifth film including silicon oxide.
 16. A method of fabricating asemiconductor integrated circuit chip comprising: preparing a waferincluding a dielectric substrate that is transparent to light and havinga front surface and a back surface, a first film formed on the backsurface of the dielectric substrate, a second film formed on the firstfilm, a third film formed on the second film, and a fourth film formedon the front surface, the second film having a lower refractive indexthan the first and third films; forming circuitry in and on the fourthfilm, and dicing the wafer into chips; and illuminating the third filmby sensing light for detection of the wafer, during said forming,wherein the first film comprises polysilicon, the second film comprisessilicon nitride, and the third film comprises polysilicon, and whereinthe first film has a thickness of substantially 42 nm, the second filmhas a thickness of substantially 80 nm, and the third film has athickness of substantially 42 nm.
 17. A method of fabricating asemiconductor integrated circuit chip comprising: preparing a waferincluding a dielectric substrate that is transparent to light and havinga front surface and a back surface, a first film formed on the backsurface of the dielectric substrate, a second film formed on the firstfilm, a third film formed on the second film, and a fourth film formedon the front surface, the second film having a lower refractive indexthan the first and third films; forming circuitry in and on the fourthfilm, and dicing the wafer into chips; and illuminating the third filmby sensing light for detection of the wafer, during said forming,wherein the dielectric substrate has side surfaces, the method furthercomprising forming a sixth film covering the side surfaces, wherein thesixth film comprises polysilicon and silicon nitride, and wherein thesixth film also covers the back surface of the dielectric substrate, thesixth film disposed between the back surface and the first film.